For the purpose of removing substances usually eliminated with urine and for the purpose of withdrawing fluid, use is made of various methods for machine-aided blood treatment in acute or chronic kidney failure. In the case of hemodialysis (HD), a patient's blood is conveyed in an extracorporeal blood circuit through one chamber of a dialyzer divided by a semipermeable membrane into two chambers, whilst a dialyzing fluid flows through the other chamber. A diffusive substance exchange essentially takes place via the membrane of the dialyzer. Only a convective substance exchange is present in the case of hemofiltration (HF). Hemodiafiltration (HDF) is a combination of the two methods.
The fluid withdrawn via the membrane of the dialyzer from the blood flowing in the extracorporeal blood circuit is referred to as ultrafiltrate. In the case of hemodiafiltration, a part of the ultrafiltrate withdrawn through the membrane of the dialyzer is replaced by a sterile substitution fluid, which is fed back to the extracorporeal blood circuit either upstream of the dialyzer (pre-dilution) or downstream of the dialyzer (post-dilution). Pre- and post-dilution can also take place at the same time. The sterile substituate, which is fed to the blood circuit, can be prepared online from the dialyzing fluid. The quantity of substituate that is fed in a specific period to the blood flowing in the extracorporeal blood circuit is referred to as the substituate rate. The rate at which fluid is withdrawn from the patient is referred to as the net withdrawal rate, as well as the ultrafiltration rate in general linguistic usage. The latter emerges as the difference between the substitution rate and the rate of the fluid displacement across the membrane.
It has been shown that an HDF blood treatment in which a post-dilution takes place has a higher efficiency, with an identical substituate rate, than a treatment in which a pre-dilution takes place. The higher cleaning capacity with the post-dilutive addition of substitution fluid compared to the pre-dilutive addition of substitution fluid is due to the fact that the filtrate is obtained completely from the blood to be cleaned in the case of post-dilution, whereas in the case of pre-dilution the blood diluted with substituate flows into the dialyzer (DE 103 55 042 B3).
The flow resistance of the membrane of the dialyzer is of importance for an extracorporeal blood treatment. With an excessively high flow resistance, the blood to be cleaned in the extracorporeal blood circuit may possibly not be able to be conveyed at the required delivery rate, as a result of which the effectiveness of the blood treatment is reduced. A greatly increased flow resistance of the dialyzer can even lead to complete blocking-up of the membrane. The treatment is then interrupted and, the whole blood hose system may have to be replaced (DE 103 55 042 B3). The effectiveness of the blood treatment itself, with an unchanged delivery rate, is reduced by the influence of the exchange surfaces of the membrane, in particular also the pores of the membrane itself.
DE 103 55 042 B3 describes a method for detecting disruptions of the blood flow in an extracorporeal blood circuit during an extracorporeal blood treatment with an extracorporeal blood treatment apparatus. The known method is based on the analysis of an oscillating pressure signal propagated in the extracorporeal blood circuit which is measured and analyzed, the phase angle of at least one harmonic of the pressure signal being determined. A disruption of the blood flow in the extracorporeal blood circuit is detected on the basis of the change in the phase angle of the at least one harmonic.
A method of detecting the clogging of the membrane of a dialyzer is known from WO 2004/073772 A1. The known method is based on an analysis of the frequency spectrum of a pressure signal transmitted via the dialyzer. During the blood treatment, the pressure conditions in the extracorporeal blood circuit and/or in the dialyzing fluid system are continuously monitored. While the pressure in the extracorporeal circuit and/or the dialyzing substituate rate and the ultrafiltration rate remain unchanged.
US 2002/0174721 A1 and U.S. Pat. No. 6,623,443 BI describe methods for detecting stenoses in a hose line system of an extracorporeal blood circuit. The two methods are based on an analysis of pressure pulses which are detected in the extracorporeal blood circuit. The method known from US 2002/0174721 A1 makes provision for analyzing the frequency spectrum of the pressure pulses and determining the attenuation of at least one harmonic of the pressure signal, it being concluded that there is a stenosis if there is a change in the attenuation. A change in the substituate or ultrafiltration rate is not taken into account in the analysis of the pressure pulses.